The Organic Light Emitting Display (OLED) possesses many outstanding properties of self-illumination, low driving voltage, high luminescence efficiency, short response time, high clarity and contrast, near 180° view angle, wide range of working temperature, applicability of flexible display and large scale full color display. The OLED is considered as the most potential display device.
The OLED is a current driving element. When the electrical current flows through the organic light emitting diode, the organic light emitting diode emits light, and the brightness is determined according to the current flowing through the organic light emitting diode itself. Most of the present Integrated Circuits (IC) only transmit voltage signals. Therefore, the OLED pixel driving circuit needs to accomplish the task of converting the voltage signals into the current signals. The traditional OLED pixel driving circuit generally is 2T1C, which is a structure comprising two thin film transistors and one capacitor to convert the voltage into the current.
As shown in FIG. 1, which is a 2T1C pixel driving circuit employed for OLED, comprising a first thin film transistor T10, a second thin film transistor T20 and a capacitor C10. The first thin film transistor T10 is a switch thin film transistor, and the second thin film transistor T20 is a drive thin film transistor, and the capacitor C10 is a storage capacitor. Specifically, a gate of the first thin film transistor T10 receiving a scan signal Scan, and a drain receiving a data signal Data, and a source being electrically coupled to a gate of the second thin film transistor T20 and one end of the capacitor C10; a drain of the second thin film transistor T20 receiving the power source voltage OVDD, and a source being electrically coupled to an anode of the organic light emitting diode D10; a cathode of the organic light emitting diode D10 receiving a common ground voltage OVSS; one end of the capacitor C10 being electrically coupled to the gate of the second thin film transistor T20, and the other end being electrically coupled to a source of the second thin film transistor T20. As the OLED displays, the scan signal Scan controls the first thin film transistor T10 to be activated, and the data signal Data enters the gate of the second thin film transistor T20 and the capacitor C10 via the first thin film transistor T10. Then, the first thin film transistor T10 is deactivated. With the storage function of the capacitor C10, the gate voltage of the second thin film transistor T20 can remain to hold the data signal voltage to make the second thin film transistor T20 to be in the conducted state to drive the current to enter the organic light emitting diode D10 via the second thin film transistor T20 and to drive the organic light emitting diode D10 to emit light.
The formula of the current flowing through the thin film transistor and the organic light emitting diode according to calculation is:IOLED=K×(Vgs−Vth)2 
wherein IOLED represents a current flowing through the driving thin film transistor and the organic light emitting diode, and K is an intrinsic conductive factor of the driving thin film transistor, and Vgs represents a voltage difference between the gate and the source of the driving thin film transistor, and Vth represents a threshold voltage of the driving thin film transistor. Accordingly, the value of the IOLED is relevant with the threshold voltage Vth of the driving thin film transistor.
The structure of the foregoing traditional OLED pixel driving circuit is simpler and does not possess compensation function, and thus lots of defects exist, wherein the more obvious one is: due to the nonconsistency in the manufacture process of the thin film transistor, the threshold voltages of the driving thin film transistors of all pixels in the OLED display device are not consistent; and the long period of working time will age the material of the driving thin film transistors to lead to the drifts of the threshold voltages of the driving thin film transistors and the to cause the phenomenon of the nonuniform display.
FIG. 2 shows a 3T1C structure OLED pixel driving circuit with compensation function according to prior art. On the basis of the traditional OLED pixel driving circuit shown in FIG. 1, a third thin film transistor T30 is added, and a gate of the third thin film transistor T30 receives a sensing control signal Sense, and a source is electrically coupled to a source of the second thin film transistor T20, and a drain is electrically coupled to a analog to digital converter and receives a reference voltage signal Vref, and a data signal Data is provided by a digital to analog converter. The OLED pixel driving circuit of 3T1C structure can sense the threshold voltage Vth of the driving thin film transistor, and compensate the Vth value to the data signal Data, which can eliminate the influence of the threshold voltage Vth of the driving thin film transistor to the current IOLED flowing through the organic light emitting diode, and thus to make the display uniform to improve the image quality. However, the OLED pixel driving circuit of 3T1C structure has drawbacks:
1. the reference voltage signal Vref provides the reference voltages to the respective pixels, and the wiring causes the aperture of the pixel decrease.
2. The generation of the reference voltage signal Vref increases the channel amount of the driving IC to increase the manufacture cost.